JPH07139940A - Focusing type displacement measuring apparatus - Google Patents

Abstract

PURPOSE:To make it possible to measure displacement highly accurately even in the measurement of surface roughness and the like including a high-frequency component by converting the level of a focus error signal into the amount of displacement, and adding the result to the amount of displacement outputted from an encoder. CONSTITUTION:In a servo system 20, a focus error signal is generated with a focus-error-signal forming circuit 21 from the outputs of two photodetectors 9a and 9b. The signal is supplied into the voice coil of an actuator through a phase correcting/driving circuit 22. In a scale-signal processing system 27, the output of an encoder 15 is counted with a counter 25, and the amount of displacement is outputted. The level of the focus error signal obtained in a forming circuit 21 undergoes digital conversion with 11 an A/D converter 23. The result is converted into the amount of displacement with a converting circuit 24. The converted focus error signal is added to the amount of displacement outputted from the counter 25 in an adding circuit 26 as the error correcting value. Then, the amount of the displacement, whose error is corrected, is outputted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focusing type displacement measuring device using an optical pickup technique.

[0002]

2. Description of the Related Art A light beam such as a laser beam is condensed on a measuring surface, and the reflected light is received to obtain a focus error signal by an astigmatism method or the like, thereby controlling an actuator for driving an objective lens. A focus displacement measuring device equipped with a servo mechanism is known. The displacement measurement is performed by an encoder including a linear scale integrally attached to the actuator and a detector facing the linear scale.

[0003]

The conventional in-focus type displacement measuring device of this type has a problem that the followability of the servo system is poor in, for example, surface roughness measurement, and high-speed measurement and high-precision measurement are difficult. It was That is, when an output including a high frequency component is obtained, there is a limit to the high speed performance of the servo system.
For this reason, the objective lens cannot move to the zero-cross position of the S curve, and a displacement measurement error occurs.

It is an object of the present invention to provide a focusing type displacement measuring apparatus which compensates for such a measurement error and enables high speed measurement and high precision measurement.

[0005]

An in-focus type displacement measuring device according to the present invention outputs a focus error signal by collecting a light beam on a measurement surface through an objective lens and receiving the reflected light. An optical pickup including an optical system that outputs a signal and an actuator that drives the objective lens, a servo mechanism that controls and drives the actuator by obtaining a focus error signal from the output signal of the optical pickup, and is attached to the actuator. Encoder for detecting the displacement amount thereof, signal processing means for processing the output signal of the encoder and outputting the displacement of the measurement surface, and converting the level of the focus error signal into the displacement amount and outputting by the encoder And a means for adding up to the amount of displacement to be performed.

[0006]

In the present invention, focusing on the point that the focus error signal level changes according to the amount of displacement error to be measured, the focus error signal level is converted into a displacement amount, which is used as a correction value. Is added to the displacement amount that is the output of. As a result, even when the objective lens of the optical pickup does not accurately follow the so-called S-curve zero-cross position due to the limit of the performance of the servo system, the measurement value by the encoder is corrected and the accurate displacement is required. That is, highly accurate displacement measurement is possible even in the surface roughness measurement including high frequency components.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a displacement measuring device body according to an embodiment of the present invention. A semiconductor laser 1 is used as a light source of an optical pickup, and its output light beam has a polarization beam splitter 2
Through the 1/4 wavelength plate 3, the collimator lens 4, and the objective lens 5, and is focused on the measurement surface 6. The reflected light beam from the measurement surface 6 travels backward and passes through the polarization beam splitter 2, passes through the imaging lens 7, and is split into two by the beam splitter 8.
Light is received by a and 9b. Stops 10a and 10b are provided in front of the light receiving elements 9a and 9b, respectively.

The objective lens 5 is driven in the optical axis direction by an actuator 13 including a movable coil 11 and a magnet 12. An encoder 15 including a linear scale integrally attached to an actuator 13 that drives the objective lens and a fixed detector 14 facing the linear scale is provided.

FIG. 2 is a functional block diagram of the servo system 20 for controlling the actuator and the scale signal processing system 27 from the encoder in this embodiment. Servo system 20
Then, the focus error signal generation circuit 21 generates a focus error signal from the outputs of the two light receiving elements 9a and 9b, and this is supplied to the voice coil of the actuator via the phase correction / drive circuit 22. In the scale signal processing system 27, the output of the encoder 15 is the counter 25.
Is counted and the displacement amount is output. In this embodiment, the level of the focus error signal obtained from the focus error signal generation circuit 21 is A / D converter 23.
Is converted into a digital value by the conversion circuit 24 and converted into a displacement amount by the conversion circuit 24. The converted focus error signal is added as an error correction value to the output displacement amount of the counter 25 in the summing circuit 26, and the error corrected displacement amount is output.

FIG. 3 is a more specific example of the functional blocks of FIG. The outputs of the two light receiving elements 9a and 9b are converted into voltages by the current-voltage conversion circuits 31a and 31b, respectively, and amplified by the amplifiers 32a and 32b. The subtracted circuit 33 and the added circuit 34 add the amplified signals A and B to the difference (B-
A) and the sum (A + B) are obtained, and these are divided by the division circuit 35 to obtain (B−A) / (A + B). The subtraction circuit 33, the addition circuit 34, and the division circuit 35 are the main body of the S-curve signal (focus error signal) generation circuit. 4 and 5 show the relationship between the outputs of the two light receiving elements 9a and 9b and the focus error signal obtained by calculating the outputs. This focus error signal is supplied to the voice coil 39 of the actuator via the amplifier 36, the phase correction circuit 37, and the drive circuit 38.

In the scale signal processing system, the output of the encoder 15 is counted by the counter 42 via the interpolation circuit 41, and the output is sent to the CPU 43 via the bus. The CPU 43 includes a timer 44 and a RAM 4
5, ROM 46, keyboard 47, display device 48, data output device 49, etc. are connected. With this signal processing system, the displacement of the measurement surface can be obtained from the encoder output in the focused state. The focus error signal obtained from the division circuit 35 of the servo system is converted into digital data by the A / D converter 23 and sent to the signal processing system. Since the focus error signal level and the displacement amount are associated with each other as shown in FIG.
It is stored in the OM 46. The displacement amount output from the counter 42 and the conversion correction value from the ROM 46 corresponding to the output obtained from the A / D converter 23 are controlled by the timer 44, for example, by the CPU 43.
Is read, and these are added up and output as a displacement amount corrected for error. Thus, according to this embodiment,
The displacement measurement error that occurs because the servo system cannot follow is
An accurate displacement amount is obtained by being corrected based on the focus error signal.

[0012]

As described above, according to the present invention, there is provided a focusing type displacement measuring device utilizing an optical pickup technique, which is capable of correcting a measurement error in surface roughness measurement including a high frequency component. It is possible to provide a measuring device that enables highly accurate measurement.

[Brief description of drawings]

FIG. 1 shows a configuration of a displacement measuring apparatus main body according to an embodiment of the present invention.

FIG. 2 shows functional blocks of a servo system and a signal processing system of the embodiment.

FIG. 3 shows a specific configuration example of the same functional block.

FIG. 4 shows a relationship between outputs and displacements of two light receiving elements.

FIG. 5 shows a relationship between a focus error signal and displacement.

FIG. 6 shows how a focus error signal is converted into a displacement amount.

[Explanation of symbols]

1 ... Semiconductor laser, 2 ... Polarization beam splitter, 3 ... 1
/ 4 wavelength plate, 5 ... Objective lens, 6 ... Measuring surface, 7 ... Imaging lens, 8 ... Beam splitter, 9a, 9b ... Light receiving element,
10a, 10b ... Aperture, 13 ... Actuator, 15 ...
Encoder, 21 ... Focus error signal generation circuit, 2
2 ... Phase correction / driving circuit, 23 ... A / D converter, 2
4 ... Conversion circuit, 25 ... Counter, 26 ... Summing circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masa Iwamoto 1-20-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa Mitutoyo Co., Ltd.

Claims (1)

[Claims] 1. An optical system including an optical system for condensing a light beam on a measurement surface through an objective lens and receiving reflected light thereof to output an output signal for obtaining a focus error signal, and an actuator for driving the objective lens. An optical pickup, a servo mechanism that obtains a focus error signal from the output signal of the optical pickup, and controls and drives the actuator accordingly, an encoder that is attached to the actuator to detect the displacement amount, and an output signal of the encoder. Signal processing means for processing the output of the displacement of the measurement surface, and means for converting the level of the focus error signal into a displacement amount and adding the displacement amount output by the encoder. Focusing type displacement measuring device.